The production of carbon fibers generally consists of preoxidizing organic fibers (e.g. polycrylonitrile fibers or cellulose fibers) in an oxidizing atmosphere to render them flame-retardant, and feeding the preoxidized fibers into a carbonizing furnace where they are carbonized in an inert gas atmosphere or a non-oxidizing atmosphere at a temperature of 300.degree. C. or higher. In this carbonizing step, the preoxidized organic fibers are thermally decomposed into carbon fibers. The carbonization is usually effected at a temperature between 300.degree. and 1,500.degree. C., sometimes higher than 1,500.degree. C., and if necessary, at the graphitization temperature of 2,000.degree. C. or more (see U.S. Pat. Nos. 4,073,870 and 4,321,446).
The carbon fibers produced by the above described conventional method has very low strength and ductility due not only to internal defects from microvoids but also to surface defects such as cracks. Therefore, to produce carbon fibers of high performance, the presence of surface defects must be minimized. In the carbonizing step, the preoxidized fibers release various decomposition products as they are carbonized at increasing temperatures, and the release of most decomposition products is known to occur in a temperature range of 300.degree. to 900.degree. C. The decomposition products formed in this temperature range, for example, HCN, NH.sub.3, CO, H.sub.2, H.sub.2 O, CH.sub.4, CO.sub.2 and higher molecular weight saturated and unsaturated hydrocarbons having 3 to 7 carbon atoms are gaseous under the temperature conditions where they are produced. However, in a vertical carbonizing furnace where preoxidized fibers are guided down through a heating chamber in which the temperature increases from the top to bottom, the gaseous decomposition products (hereunder decomposition gases) are carried by the ascending gas stream into the low-temperature zone of the furnace where the higher molecular weight hydrocarbons are cooled to form a tar mist. Part of the decomposition products now in the form of a tar mist is deposited on the inner surface of the furnace wall or the fiber surfaces. The sticky tar mist on the wall surfaces catches fiber fuzz adrift in the furnace and grows during continuous furnace operation. Ultimately it contacts and damages the surface of the fiber passing through the furnace or partially obstructs the passage of the fibers to upset the uniform flow of the gas stream. If the contact between the fibers and the tar mist is extreme, the individual filaments stick to each other, and the buildup of tar mist at elevated temperatures causes surface defects that greatly reduce the strength and ductility of the carbon fiber product. Furthermore, decomposition gases such as H.sub.2 O, CO.sub.2 and CO lower the fiber strength appreciably when they contact the fibers in the high-temperature zone of the furnace.